add dropdown component

lib/ui.js

@@ -1,6 +1,7 @@
 import { createIcon, resolveIcon } from "./ui/icon";
 import { createCheckbox, resolveCheckbox } from "./ui/checkbox";
 import { setTooltip, resolveTooltip } from "./ui/tooltip";
+import Dropdown from "./ui/dropdown";
 
 export {
   // icon
@@ -11,5 +12,7 @@ export {
   resolveCheckbox,
   // tooltip
   setTooltip,
-  resolveTooltip
+  resolveTooltip,
+  // dropdown
+  Dropdown
 }